(1) Field Of The Invention
The present invention generally relates to antenna systems.
(2) Description of Prior Art
Prior art antenna systems comprise a plurality of antenna elements that receive electromagnetic waves incident on the antenna elements. The dynamic range of the amplitudes of such electromagnetic waves is relatively large. The electromagnetic waves may emanate from a plurality of sources that may be non-cooperative thereby making it difficult, if not impossible, for the antenna system to effect a favorable dynamic range of incident electromagnetic waves. The aforementioned prior art antenna systems typically comprise analog signal paths or channels between the antenna elements and other signal processing components. Such analog signal paths may effect particular signal processing functions such as filtering, frequency or phase shifting, amplification, etc. However, the dynamic-range limitations of these analog signal paths can cause deleterious effects on the received signals. Furthermore, the dynamic range of the analog signal channels cannot be easily or inexpensively increased because the overall dynamic range of analog signal path depends on the tolerance and operational limitations of the individual analog signal components within the analog signal path.
Simultaneous multi-functional antenna use may be implemented by means of a wide-band digital interface that permits high spurious-free dynamic range digital (i) analysis of received signals, or (ii) synthesis of transmitted signals. Wide-band analog signal processing components are incapable of achieving the same levels of high spurious-free dynamic range operation as digital signal processing objects. Furthermore, it is unlikely that the linearity of essential analog components will ever increase substantially. This is because component linearity is intimately linked to basic semiconductor physics. However, high spurious-free dynamic range is essential for wide-band multi-functional operation.
When the dynamic range of amplitudes of the incident electromagnetic waves cover a relatively large dynamic range of amplitudes, one or more of the electromagnetic waves may be lost as a result of the non-linearity of the analog signal paths. Intermodulation distortion, weak-signal suppression, spurious-response generation, and other performance-limiting effects may occur when a combination of strong and weak signals are present simultaneously in a non-linear part of an analog signal path. This dynamic-range problem is often referred to as the xe2x80x9cnear-farxe2x80x9d problem that arises when signals arriving from a xe2x80x9cnearxe2x80x9d source and a xe2x80x9cfarxe2x80x9d source differ significantly in amplitude and must be processed by the same system.
What is needed is an antenna system that has a relatively improved spurious free dynamic range.
Therefore, it is an object of the present invention to circumvent the inadequate linearity of analog components and to provide an antenna system that addresses the foregoing problems.
It is another object of the present invention to eliminate adjustable phase shifters and attenuators on the antenna elements so as to stabilize the reflection coefficient of the antenna array over time and improves the stealth quality of the receiving system to electromagnetic probes, e.g., radar.
It is a further object of the present invention to introduce a locally generated wide-band signal observable into the phase center of each antenna element wherein the observable is used in the digital domain to facilitate a continuous calibration of the transfer function of each and every analog signal path of each antenna element so that coherent digital combining may take place in the absence of a desired signal.
Another object of the present invention is provide an antenna system that can provide stealth characteristics while providing a wide-band receiving capability in which multiple, simultaneous, individual beams can be formed using true time delay for each signal of interest in the wide band of operation.
It is a further object of the present invention to provide an antenna system that has a relatively high spurious-free dynamic range so as to provide the ability to simultaneously receive and process signals of widely differing amplitudes and bandwidths and improves resistance to jamming.
It is another object of the present invention to provide an antenna system that is relatively more stable over time in comparison to analog components and processes of conventional antenna systems which may be sensitive to temperature and component aging.
It is a further object of the present invention to provide an antenna system that exhibits improved reliability and life-cycle cost in comparison to conventional systems.
Other objects and advantages of the present invention will be apparent to one of ordinary skill in the art in light of the ensuing description of the present invention.
The present invention is directed to an antenna system that utilizes a wide-band observable signal to time-tag arriving signals at the phase center of each element of a multi-element antenna array in such a manner that the true time of arrival of these signals becomes a part of the digital representation of each signal sample. In accordance with the present invention, the antenna system of the present invention utilizes a locally generated observable signal that provides timing and other information useful for coherently combining signal components received independently at different antenna elements. Specifically, the observable signal is used to quantify the time-varying differential delay associated with each analog signal path between the phase center of each antenna element and a corresponding analog-to-digital converter. The observable signal is added to the analog signal path of each antenna element near the phase center of the antenna and is sampled with the unknown signal by the analog-to-digital converter associated with the analog signal path. The observable signal is constructed so that it can be separated from the unknown signal by correlation with a replica in order to characterize the transfer function associated with each analog signal path. Thus, the observable signal provides information about the receiving system that allows a digital signal processor to coherently and constructively combine signal components that are received by independent antenna elements.
In one aspect, the present invention is directed to a method of processing signals received by an antenna system comprising an array of antenna elements for receiving signals. Each antenna element has a phase center. The method comprises the steps of (a) generating an observable signal that contains a low-frequency component and a high-frequency component, (b) summing the high-frequency component with the signal received by each antenna element near the phase center of each antenna element to form a plurality of sum signals, (c) feeding each sum signal into an analog signal path that modifies the sum signal, (d) converting each modified sum signal into a digital sum signal, (e) processing the digital sum signals with the low frequency component of the observable signal to (i) remove the high frequency component of the observable signal, (ii) normalize the effects of the signal transfer characteristics of the analog signal paths on the digital sum signals, (iii) synchronously re-sample all digital sum signals and (iv) differentially time-reference the digital sum signals to the phase center of the corresponding antenna elements so as to provide a plurality of processed digital signals wherein each processed digital signal is a low-frequency replica of the signal received by a corresponding antenna element, and (f) combining the processed digital signals into a single composite signal.
In a related aspect, the present invention is directed to an antenna system, comprising (a) an array of antenna elements for receiving signals, each antenna element having a phase center, (b) a circuit for generating an observable signal that contains a low-frequency component and a high-frequency component, (c) a plurality of signal processing channels, each channel comprising an input for receiving a signal received from a corresponding antenna element, a summing circuit located at each element near the phase center of the corresponding antenna element for summing the high-frequency component of the observable signal with the received signal to form a sum signal, an analog signal path for modifying the sum signal, an analog-to-digital converter for converting each modified sum signal into a digital sum signal, and a digital signal processor having inputs for receiving the digital sum signal and the low-frequency component, the digital signal processor configured to (i) remove the high frequency component from the digital sum signal, (ii) normalize the effects of the signal transfer characteristics of the analog signal path on the digital sum signal, (iii) re-sample the digital sum signal synchronously with the digital signal processors of the other signal processing channels, and (iv), differentially time-reference each re-sampled digital sum signal to the phase center of the corresponding antenna elements so as to provide a processed digital sum signal that is a low-frequency replica of the signal received by the corresponding antenna element, (d) a control circuit for effecting synchronous operation of the digital signal processors of the signal processing channels, and (e) a system digital signal processor for combining the processed digital sum signals outputted by the signal processing channels into a single composite signal.